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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * accompanied this code).
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package com.nulldev.util.internal.backport.optionals;

import java.util.NoSuchElementException;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Stream;

import com.nulldev.util.internal.backport.concurrency9.Objects;

/**
 * A container object which may or may not contain a non-{@code null} value. If
 * a value is present, {@code isPresent()} returns {@code true}. If no value is
 * present, the object is considered <i>empty</i> and {@code isPresent()}
 * returns {@code false}.
 *
 * <p>
 * Additional methods that depend on the presence or absence of a contained
 * value are provided, such as {@link #orElse(Object) orElse()} (returns a
 * default value if no value is present) and {@link #ifPresent(Consumer)
 * ifPresent()} (performs an action if a value is present).
 *
 * <p>
 * This is a <a href="
 * {@docRoot}/java.base/java/lang/doc-files/ValueBased.html">value-based</a> class;
 * programmers should treat instances that are {@linkplain #equals(Object)
 * equal} as interchangeable and should not use instances for synchronization,
 * or unpredictable behavior may occur. For example, in a future release,
 * synchronization may fail.
 *
 * @apiNote {@code Optional} is primarily intended for use as a method return
 *          type where there is a clear need to represent "no result," and where
 *          using {@code null} is likely to cause errors. A variable whose type
 *          is {@code Optional} should never itself be {@code null}; it should
 *          always point to an {@code Optional} instance.
 *
 * @param <T> the type of value
 * @since 1.8
 */
public final class Optional<T> {
	/**
	 * Common instance for {@code empty()}.
	 */
	private static final Optional<?> EMPTY = new Optional<>(null);

	/**
	 * If non-null, the value; if null, indicates no value is present
	 */
	private final T value;

	/**
	 * Returns an empty {@code Optional} instance. No value is present for this
	 * {@code Optional}.
	 *
	 * @apiNote Though it may be tempting to do so, avoid testing if an object is
	 *          empty by comparing with {@code ==} or {@code !=} against instances
	 *          returned by {@code Optional.empty()}. There is no guarantee that it
	 *          is a singleton. Instead, use {@link #isEmpty()} or
	 *          {@link #isPresent()}.
	 *
	 * @param <T> The type of the non-existent value
	 * @return an empty {@code Optional}
	 */
	public static <T> Optional<T> empty() {
		@SuppressWarnings("unchecked")
		Optional<T> t = (Optional<T>) EMPTY;
		return t;
	}

	/**
	 * Constructs an instance with the described value.
	 *
	 * @param value the value to describe; it's the caller's responsibility to
	 *              ensure the value is non-{@code null} unless creating the
	 *              singleton instance returned by {@code empty()}.
	 */
	private Optional(T value) {
		this.value = value;
	}

	/**
	 * Returns an {@code Optional} describing the given non-{@code null} value.
	 *
	 * @param value the value to describe, which must be non-{@code null}
	 * @param <T>   the type of the value
	 * @return an {@code Optional} with the value present
	 * @throws NullPointerException if value is {@code null}
	 */
	public static <T> Optional<T> of(T value) {
		return new Optional<T>(Objects.requireNonNull(value));
	}

	/**
	 * Returns an {@code Optional} describing the given value, if non-{@code null},
	 * otherwise returns an empty {@code Optional}.
	 *
	 * @param value the possibly-{@code null} value to describe
	 * @param <T>   the type of the value
	 * @return an {@code Optional} with a present value if the specified value is
	 *         non-{@code null}, otherwise an empty {@code Optional}
	 */
	@SuppressWarnings("unchecked")
	public static <T> Optional<T> ofNullable(T value) {
		return value == null ? (Optional<T>) EMPTY : new Optional<>(value);
	}

	/**
	 * If a value is present, returns the value, otherwise throws
	 * {@code NoSuchElementException}.
	 *
	 * @apiNote The preferred alternative to this method is {@link #orElseThrow()}.
	 *
	 * @return the non-{@code null} value described by this {@code Optional}
	 * @throws NoSuchElementException if no value is present
	 */
	public T get() {
		if (value == null) {
			throw new NoSuchElementException("No value present");
		}
		return value;
	}

	/**
	 * If a value is present, returns {@code true}, otherwise {@code false}.
	 *
	 * @return {@code true} if a value is present, otherwise {@code false}
	 */
	public boolean isPresent() {
		return value != null;
	}

	/**
	 * If a value is not present, returns {@code true}, otherwise {@code false}.
	 *
	 * @return {@code true} if a value is not present, otherwise {@code false}
	 * @since 11
	 */
	public boolean isEmpty() {
		return value == null;
	}

	/**
	 * If a value is present, performs the given action with the value, otherwise
	 * does nothing.
	 *
	 * @param action the action to be performed, if a value is present
	 * @throws NullPointerException if value is present and the given action is
	 *                              {@code null}
	 */
	public void ifPresent(Consumer<? super T> action) {
		if (value != null) {
			action.accept(value);
		}
	}

	/**
	 * If a value is present, performs the given action with the value, otherwise
	 * performs the given empty-based action.
	 *
	 * @param action      the action to be performed, if a value is present
	 * @param emptyAction the empty-based action to be performed, if no value is
	 *                    present
	 * @throws NullPointerException if a value is present and the given action is
	 *                              {@code null}, or no value is present and the
	 *                              given empty-based action is {@code null}.
	 * @since 9
	 */
	public void ifPresentOrElse(Consumer<? super T> action, Runnable emptyAction) {
		if (value != null) {
			action.accept(value);
		} else {
			emptyAction.run();
		}
	}

	/**
	 * If a value is present, and the value matches the given predicate, returns an
	 * {@code Optional} describing the value, otherwise returns an empty
	 * {@code Optional}.
	 *
	 * @param predicate the predicate to apply to a value, if present
	 * @return an {@code Optional} describing the value of this {@code Optional}, if
	 *         a value is present and the value matches the given predicate,
	 *         otherwise an empty {@code Optional}
	 * @throws NullPointerException if the predicate is {@code null}
	 */
	public Optional<T> filter(Predicate<? super T> predicate) {
		Objects.requireNonNull(predicate);
		if (!isPresent()) {
			return this;
		} else {
			return predicate.test(value) ? this : empty();
		}
	}

	/**
	 * If a value is present, returns an {@code Optional} describing (as if by
	 * {@link #ofNullable}) the result of applying the given mapping function to the
	 * value, otherwise returns an empty {@code Optional}.
	 *
	 * <p>
	 * If the mapping function returns a {@code null} result then this method
	 * returns an empty {@code Optional}.
	 *
	 * @apiNote This method supports post-processing on {@code Optional} values,
	 *          without the need to explicitly check for a return status. For
	 *          example, the following code traverses a stream of URIs, selects one
	 *          that has not yet been processed, and creates a path from that URI,
	 *          returning an {@code Optional<Path>}:
	 *
	 *          <pre>{@code
	 *     Optional<Path> p =
	 *         uris.stream().filter(uri -> !isProcessedYet(uri))
	 *                       .findFirst()
	 *                       .map(Paths::get);
	 * }</pre>
	 *
	 *          Here, {@code findFirst} returns an {@code Optional<URI>}, and then
	 *          {@code map} returns an {@code Optional<Path>} for the desired URI if
	 *          one exists.
	 *
	 * @param mapper the mapping function to apply to a value, if present
	 * @param <U>    The type of the value returned from the mapping function
	 * @return an {@code Optional} describing the result of applying a mapping
	 *         function to the value of this {@code Optional}, if a value is
	 *         present, otherwise an empty {@code Optional}
	 * @throws NullPointerException if the mapping function is {@code null}
	 */
	public <U> Optional<U> map(Function<? super T, ? extends U> mapper) {
		Objects.requireNonNull(mapper);
		if (!isPresent()) {
			return empty();
		} else {
			return Optional.ofNullable(mapper.apply(value));
		}
	}

	/**
	 * If a value is present, returns the result of applying the given
	 * {@code Optional}-bearing mapping function to the value, otherwise returns an
	 * empty {@code Optional}.
	 *
	 * <p>
	 * This method is similar to {@link #map(Function)}, but the mapping function is
	 * one whose result is already an {@code Optional}, and if invoked,
	 * {@code flatMap} does not wrap it within an additional {@code Optional}.
	 *
	 * @param <U>    The type of value of the {@code Optional} returned by the
	 *               mapping function
	 * @param mapper the mapping function to apply to a value, if present
	 * @return the result of applying an {@code Optional}-bearing mapping function
	 *         to the value of this {@code Optional}, if a value is present,
	 *         otherwise an empty {@code Optional}
	 * @throws NullPointerException if the mapping function is {@code null} or
	 *                              returns a {@code null} result
	 */
	public <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
		Objects.requireNonNull(mapper);
		if (!isPresent()) {
			return empty();
		} else {
			@SuppressWarnings("unchecked")
			Optional<U> r = (Optional<U>) mapper.apply(value);
			return Objects.requireNonNull(r);
		}
	}

	/**
	 * If a value is present, returns an {@code Optional} describing the value,
	 * otherwise returns an {@code Optional} produced by the supplying function.
	 *
	 * @param supplier the supplying function that produces an {@code Optional} to
	 *                 be returned
	 * @return returns an {@code Optional} describing the value of this
	 *         {@code Optional}, if a value is present, otherwise an
	 *         {@code Optional} produced by the supplying function.
	 * @throws NullPointerException if the supplying function is {@code null} or
	 *                              produces a {@code null} result
	 * @since 9
	 */
	public Optional<T> or(Supplier<? extends Optional<? extends T>> supplier) {
		Objects.requireNonNull(supplier);
		if (isPresent()) {
			return this;
		} else {
			@SuppressWarnings("unchecked")
			Optional<T> r = (Optional<T>) supplier.get();
			return Objects.requireNonNull(r);
		}
	}

	/**
	 * If a value is present, returns a sequential {@link Stream} containing only
	 * that value, otherwise returns an empty {@code Stream}.
	 *
	 * @apiNote This method can be used to transform a {@code Stream} of optional
	 *          elements to a {@code Stream} of present value elements:
	 * 
	 *          <pre>{@code
	 *     Stream<Optional<T>> os = ..
	 *     Stream<T> s = os.flatMap(Optional::stream)
	 * }</pre>
	 *
	 * @return the optional value as a {@code Stream}
	 * @since 9
	 */
	public Stream<T> stream() {
		if (!isPresent()) {
			return Stream.empty();
		} else {
			return Stream.of(value);
		}
	}

	/**
	 * If a value is present, returns the value, otherwise returns {@code other}.
	 *
	 * @param other the value to be returned, if no value is present. May be
	 *              {@code null}.
	 * @return the value, if present, otherwise {@code other}
	 */
	public T orElse(T other) {
		return value != null ? value : other;
	}

	/**
	 * If a value is present, returns the value, otherwise returns the result
	 * produced by the supplying function.
	 *
	 * @param supplier the supplying function that produces a value to be returned
	 * @return the value, if present, otherwise the result produced by the supplying
	 *         function
	 * @throws NullPointerException if no value is present and the supplying
	 *                              function is {@code null}
	 */
	public T orElseGet(Supplier<? extends T> supplier) {
		return value != null ? value : supplier.get();
	}

	/**
	 * If a value is present, returns the value, otherwise throws
	 * {@code NoSuchElementException}.
	 *
	 * @return the non-{@code null} value described by this {@code Optional}
	 * @throws NoSuchElementException if no value is present
	 * @since 10
	 */
	public T orElseThrow() {
		if (value == null) {
			throw new NoSuchElementException("No value present");
		}
		return value;
	}

	/**
	 * If a value is present, returns the value, otherwise throws an exception
	 * produced by the exception supplying function.
	 *
	 * @apiNote A method reference to the exception constructor with an empty
	 *          argument list can be used as the supplier. For example,
	 *          {@code IllegalStateException::new}
	 *
	 * @param <X>               Type of the exception to be thrown
	 * @param exceptionSupplier the supplying function that produces an exception to
	 *                          be thrown
	 * @return the value, if present
	 * @throws X                    if no value is present
	 * @throws NullPointerException if no value is present and the exception
	 *                              supplying function is {@code null}
	 */
	public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
		if (value != null) {
			return value;
		} else {
			throw exceptionSupplier.get();
		}
	}

	/**
	 * Indicates whether some other object is "equal to" this {@code Optional}. The
	 * other object is considered equal if:
	 * <ul>
	 * <li>it is also an {@code Optional} and;
	 * <li>both instances have no value present or;
	 * <li>the present values are "equal to" each other via {@code equals()}.
	 * </ul>
	 *
	 * @param obj an object to be tested for equality
	 * @return {@code true} if the other object is "equal to" this object otherwise
	 *         {@code false}
	 */
	@Override
	public boolean equals(Object obj) {
		if (this == obj) {
			return true;
		}

		if (!(obj instanceof Optional)) {
			return false;
		}

		Optional<?> other = (Optional<?>) obj;
		return Objects.equals(value, other.value);
	}

	/**
	 * Returns the hash code of the value, if present, otherwise {@code 0} (zero) if
	 * no value is present.
	 *
	 * @return hash code value of the present value or {@code 0} if no value is
	 *         present
	 */
	@Override
	public int hashCode() {
		return Objects.hashCode(value);
	}

	/**
	 * Returns a non-empty string representation of this {@code Optional} suitable
	 * for debugging. The exact presentation format is unspecified and may vary
	 * between implementations and versions.
	 *
	 * @implSpec If a value is present the result must include its string
	 *           representation in the result. Empty and present {@code Optional}s
	 *           must be unambiguously differentiable.
	 *
	 * @return the string representation of this instance
	 */
	@Override
	public String toString() {
		return value != null ? String.format("Optional[%s]", value) : "Optional.empty";
	}

	public static <T> Optional<T> convert(final java.util.Optional<T> t) {
		return new Optional<T>(t.isPresent() ? t.get() : null);
	}
}